Intruder detection systems for home or commercial security applications often employ PIR sensors to detect the movement of heat-emitting objects within a detection area. A PIR sensor typically includes a pair of heat sensor elements. Each of the heat sensor elements comprises a pyroelectric material, or other radiation sensitive material, that generates electric charge in response to incident infrared radiation. The heat sensor elements generate oppositely poled signals. The PIR sensor includes a fresnel lens that defines the field of view of the sensor elements. The fresnel lens includes an array of sub-lenses that divides the detection area into a plurality of detection zones. The sub-lenses focus infrared radiation from each of the detection zones onto the heat sensor elements. Each of the heat sensor elements generates a signal representative of the incident infrared radiation. The PIR sensor sums the oppositely poled sensor element signals to produce a detection signal.
Signal processing circuitry associated with the PIR sensor receives the detection signal and performs appropriate amplification and filtering for presentation of the signal to a comparator circuit. The detection signal represents the difference between heat emitted by an intruder and background heat emission. If no intruder is present, the signals generated by both of the heat sensor elements will represent background heat emission and generally cancel out one another when summed. As a result, the detection signal will tend toward zero, or at least some base-line level, when no intruder is present.
As a heat-emitting object crosses from one zone to another, the amount of heat radiation received by the sensor elements will vary. In particular, boundaries between zones created by the sub-lenses will block portions of the infrared radiation emitted by the object. The sensor elements are spatially displaced relative to one another. Thus, as the object moves and interacts with zone boundaries, the sensor elements will receive radiation from a given zone in a temporally displaced manner. The summed detection signal therefore will rise and fall, taking on an alternating waveform. The frequency of the waveform is a function of the velocity of the object across the zones and the distance of the object from the sensor elements. The rise and fall of the detection signal provides an indication of movement within the detection area, whereas the amplitude of the signal gives an indication of its significance, i.e., whether the signal is indicative of a heat-emitting object that qualifies as an intruder.
To determine the significance of the detection signal, a comparator circuit is provided to compare the signal to a predetermined threshold. The threshold may take the form of a window having upper and lower thresholds. Upon a signal excursion outside of the window, i.e., above the upper threshold or below the lower threshold, a window comparator generates a signal indicative of the presence of an intruder. The detection system then initiates a response such as the activation of an alarm and/or the dispatch of security personnel.
The avoidance of false alarms is a concern in any detection system. Also important is the ability to detect the presence of an intruder under a variety of conditions. Due to a number of variations, however, the comparison of the signal to a predetermined threshold can result in false alarms or the failure to detect intruders. Such variations include changes in environmental conditions existing in the detection area, various features of an object entering the detection area, varying characteristics of the signal processing circuitry, or a combination of the above factors.
As an example, the detection signal can vary in amplitude as a function of intruder emission relative to an ambient temperature existing in the detection area. Specifically, as the ambient temperature varies, the background heat emission similarly varies. The result is a variation in the amplitude of the intruder detection signal. As the ambient temperature approaches human body temperature the difference in temperature between background and an intruder will decrease. Consequently, the amplitude of the detection signal decreases significantly. If the threshold window is set too wide, the signal processing circuitry may fail to resolve the presence of an intruder at ambient temperatures producing smaller signal amplitudes. If the threshold window is set too narrow, however, the vulnerability of the system to false alarms increases.
The detection signal also varies as a function of the frequency of the detection signal, and thus the velocity of an object within the detection area. As the velocity of an object increases, the amplitude of the detection signal tends to decrease. When the detection signal is compared to a threshold, variations in the signal amplitude due to the velocity of an object in the detection area can result in false alarms or detection failure.
Another variation in the amplitude of the detection signal that can lead to false alarms or detection failure is the introduction of a slow ac rise or fall. Specifically, over time, the detection signal can acquire a significant, but slowly changing, increase or decrease due to factors such as changes in ambient temperature or prolonged presence of an intruder within one or more zones. The increase or decrease tends to shift the magnitude of the signal. Although the output of PIR sensor typically will be ac-coupled to a signal processing circuit, the resulting rise or fall may have the local effect of a dc offset. With such a shift, a detection signal that otherwise would not be indicative of the presence of an intruder may extend outside of a given threshold window. As a result, the detection system may register a false alarm. With a shift causing the signal to fall inside of one of the window thresholds, the detection system may fail to detect the presence of an intruder.